I have noticed that periodically when adding a transition between clips, playback is choppy/ stalled. I have tried deleting and re-adding the clips, deleting and re-adding the transitions, re-importing the thumbnails, moving the footage into a new event, and of course, restarting iMovie. Nothing I try seems to help. I have experimented with several types of transitions; it is crossfade that seems to cause the most noticeable glitch: the end of the first clip will freeze at the very end and then speed up the beginning of that which follows. When I use fade-to-black, the freezing is not quite as apparent but the black screen stays up far longer than it should if functioning normally. I attempted to tweak the transition duration as well but that actually made things worse. To reiterate, it does not happen with all clips but it is rendering otherwise complete projects unfit for viewing.

Has anyone ever had this problem? I am currently using iMovie '11 on an iMac running 10.8.2. Any suggestions would be greatly appreciated.

The projects are no longer than five minutes each. They played fine before the transitions were added but I find that once the problem has arisen (and again, it's only affecting some of them), it won't go away regardless of how the clips are edited/ removed/ re-added. All of the source material is in the same format, namely .m4v. Currently I have one transition that simply will not behave; there are six others that are functioning normally.

I don't think it's the source files. Most of my source files are MOV format. I used to experience similar issues. If I applied a transition to a PIP clip the transaction would mess up and not even show the PIP clip at all. Although the final exported movie was always fine, it just happened in iMovie.

I also tried restarting imovie etc, but I could never solve the issue either. This, along with the terrible export quality, made me finally ditch imovie and turn to FCPX, and to be honest its the best move I ever made!